Capillary-assisted evaporation characteristics of TiO₂-CNT-Cu-nanocomposite-coated plates and finned plates

The present study involves the experimental investigation of capillary-assisted evaporation on nanocomposite-coated plates and finned structured plates, which were vertically installed on the refrigerant pool. To enhance the capillary action, i.e., climbing level of the thin refrigerant film, we proposed two approaches: 1) TiO₂-CNT-Cu-nanocomposite-coated plate enhanced porosity and hydrophilicity as well as the thermophysical properties of the heat transfer surface. 2) Machined-finned structures were also developed on a bare copper plate to enlarge the contact area between the thin refrigerant film and the finned walls. A semi-flooded capillary-assisted evaporator was tested with the proposed enhanced heat transfer surfaces; herein, both the capillary-assisted evaporation and liquid natural convection evaporation occurred simultaneously. Results of wettability test on the nanocomposite-coated surfaces with distilled-water droplets revealed that the TiO₂-CNT-Cu-nanocomposite-coated surface exhibited superhydrophilic characteristic. The experimental results illustrated that both the superhydrophilic surfaces achieved by the nanocomposite coating on the bare Cu plate and the finned plates enhanced the evaporation heat transfer by the formation of the liquid thin-film on the heat transfer surface. The filling level of the refrigerant pool for the semi-flooded evaporation test plate was also considered as the main factor for optimizing the capillary-assisted evaporation. As a result, the values of the peak evaporation heat transfer for all the tested plates were determined in terms of the filling levels. It was noteworthy that at a certain filling level, the upper body of the test plate was completely occupied by the highest height of the ultra-thin refrigerant film and the underbody was fully used for the liquid natural convection. The future work will include the evaporation mechanism of the evaporation heat transfer on the nanocomposite-coated on the machined-finned structures.